The invention relates to a low-pressure mercury discharge lamp for tanning purposes, comprising
a lamp vessel which is closed in a gastight manner and filled with mercury and one or more inert gases, PA1 a luminescent screen, PA1 means for maintaining a discharge in the lamp vessel, during operation of the lamp.
Such a low-pressure mercury discharge lamp, hereinafter also referred to as lamp, is known from U.S. Pat. No. 4,703,224. This lamp comprises a lamp vessel formed from an "open" glass having a relatively high transmission for UV-radiation of a relatively short wavelength. The luminescent screen of the known lamp comprises a mixture of cerium-activated strontium magnesium aluminate, europium-activated strontium pyrophosphate and europium-activated barium pyrophosphate. The emission spectrum of this luminescent screen for wavelengths below 400 nm corresponds substantially to the spectrum of sunlight. Since the UV-radiation acting on the skin is situated mainly in this wavelength range, the emission spectrum of the known lamp has biological effects which also correspond substantially to those of sunlight. More particularly, the lamp has advantageous properties as regards tanning and thickening of the skin so that an increased resistance against reddening of the skin caused by over-exposure to sunlight is brought about. A drawback of the known lamp, however, is that the luminescent substances included in the luminescent screen demonstrate a certain degree of optical interaction with each other, so that a part of the light emitted by these luminescent substances is absorbed again by the luminescent screen. This optical interaction causes a relatively low effectiveness of the lamp. Since the degree to which optical interaction occurs depends substantially on the thickness of the luminescent screen, and in practice this thickness varies relatively substantially within a lamp, the optical interaction causes relatively large differences between the emission spectra of different parts of the luminescent screen.